You need to treat the coop too. The nasty thing with mites is they get off the birds and into the coop and live in the litter and wood. Also, DE is a good preventative, but it doesn't kill adult mites well. You need to dust with something strong like Sevin. My advice is buy some Sevin, dust all your birds. Then tear the coop apart, wash it with gentle soap and water, change the litter, and dust itall over with the Seven our better yet spray it with the Permethrin since it will get into gaps in the wood better than the Seven will. Repeat in seven to ten days because this won't kill the eggs, so you need to go again when they hatch to completely wipe them out or you will just end up reinfested.
x2...If you're not treating the coop then you're just throwing them back in there with the mites. I had a coop for my silkies that I was able to fit in my shed this winter. I dealt with mites in their crest feathers all winter long. I finally removed them from the coop, removed the coop from the shed and just set them up in a pen in the shed. they're doing much better and I haven't seen any mites. I think because they are now in the "open" rather than being in a coop where humidity and moisture can collect. But I knew I wouldn't be able to clean out that coop in the middle of winter so it's just sitting outside now until it warms up. I don't plan on using it at all actually. I may turn it into something else and remove the run and just the house part something. Who knows. But either way, you need to disinfect that coop.